The present invention relates to hydraulic rams and has a particular application in the field of telescopic hydraulic rams used to operate dump trucks and the like.
There are two basic types of telescopic rams that are used for raising and lowering the body of a dump truck. The first type of ram is known generally in the art as a displacement sealing ram, or, a rod sealing system. In this type of device the telescoping rams and cylinders are sealed from each other by means of a stationary seal at the upper end of the cylinder of the larger ram and the seal occupies part of its cylinder. This results in the reduction of ram cross-sectional area against which hydraulic fluid may act thereby reducing the available exertable force for a given diameter ram. However, this type of sealing system provides the desirable feature that because each telescoping segment is submerged in oil when the ram is collapsed any tendency for the ram to rust is vigorously resisted. This system also permits the use of an aggressive seal which avoids evidence of leakage when the ram is extended. However, if the ram is scored or otherwise damaged the aggressive seal may be reduced, or destroyed, although this drawback is somewhat mitigated because the sealing surfaces are readily inspected and their condition and the good surface finish of the rams can be readily observed.
The second type of sealing that is commonly used is the so-called "bore" sealing system. In this structure, a seal in the form of a cup is located at the lower end of each segment of ram that is to be extended. This type of system has the advantage that the moveable ram can have an active cross-sectional area which is very close to the area of the cylinder in which the ram fits, thereby maximizing the available hydraulic force for lifting purposes. Another advantage of this system, is that the non-aggressive seals can run on ram tube surface finishes that are not as demanding as those required for the displacement seal. On the other hand this system has the drawback that the seals separate the successive segments from the hydraulic oil in the chamber in which the ram segment is telescoped, with the result that the segments are not oiled to prevent rust if the seals are designed aggressively. To overcome this drawback the seals are generally designed to be non-aggressive, that is to say they are designed to permit a certain amount of oil to escape past the seal so as to lubricate the ram. However the escaped (lubricating) oil accumulates when the ram is collapsed and thereafter, when the ram is again extended after its collapse for some time, the accumulated oil spills out giving the impression that the seal may have failed. Thus although the sealing surfaces in this system are not as prone to damage as in the first discussed system, they are not readily examined and consequently, it is difficult to ascertain whether the observed spilled oil is a result of the telescoping action, or, the result of a seal having failed.